Injection molding of thermoplastic materials is widely used to produce a variety of parts. With this type of molding, complex parts having intricate shapes and requiring close dimensional tolerances can be produced. Moreover, a wide variety of materials have been successfully injection molded to form such parts.
Many forms of injection molding equipment have been developed, the most common of which utilize a stationary plastic extruder feeding through a series of runners to a plurality of mold cavities formed between a pair of separable mold dies. This apparatus includes means for clamping the dies together and for actuating the extruder to inject a "shot" of plastic into the runners and mold cavities. After the plastic has had sufficient time to solidify, the mold dies are separated and the parts ejected therefrom. Generally, when the parts are ejected from the mold dies, the runners and sprues associated therewith are also ejected. Therefafter, the solidified runner and sprue material must be separated from the molded parts and either reground and returned to the extruder or scrapped. At the same time, the separated parts must be collected and, if further assembly operations are to be performed, must be oriented before they have further operations performed thereon. While this procedure of separating the runners and sprues from the parts is costly and time consuming, it is necessary in order to achieve a reasonably high production rate from a single molding machine. Otherwise, it would only be possible to mold a single part between the mold dies, substantially reducing the production and economy of the machine. Even so, it will be appreciated that, at best, ejection molding equipment of this type is of an intermittent nature, utilizing a substantial portion of the molding cycle to solidify the plastic in the mold cavity before parts are ejected therefrom. As a result, the production and efficiency of such an ejection molding machine is less than what might otherwise be anticipated.
One attempt to simplify injection molding of thermoplastic parts has been the use of "hot runner" systems wherein the runners from the plastic extruder to the mold cavity are maintained at an elevated temperature. With this system, the plastic in the runners is maintained above the melting temperature with only the plastic in the mold cavity being solidified. Thus, only the parts are ejected from the mold cavity, with substantially no runners to remove therefrom. The step of removing the sprues and runners from the finished parts is substantially eliminated with this system. However, it has been found that hot runner molding has certain drawbacks prohibiting its utilization in the production of many types of parts. One of these drawbacks involves the heating of the runner area adjacent the mold cavity which prevents controlled and accurate cooling of the part in the mold cavity as it is solidified. As a result, it has been found that dimensional tolerances cannot be maintained in some parts due to the elevated temperature in the adjacent runner. Additionally, it has been found that certain materials, otherwise adaptable to injection, molding, deteriorate when held at elevated temperatures for extended periods of time, as in the case of hot runner molding systems.
Other forms of injection molding apparatus have been developed in an attempt to solve the foregoing problems and to reduce the amount of "idle" time necessary for the parts to solidify in the mold cavity. One form of injection molding apparatus of this type utilizes a plurality of mold assemblies mounted on a rotating table which is intermittently moved past a plastic extruder. Alternatively, the mold assemblies have been arranged in a stationary circular arrangement with the extruder rotatably mounted in the center thereof. In either arrangement, as the mold assembly comes opposite the extruder nozzle, relative motion therebetween is stopped and the extruder nozzle is moved into operating contact with the inlet to the mold cavity and the shot of thermoplastic material is injected therein. The extrusion nozzle is then retracted from the cavity inlet and is moved to the next mold assembly. With molding apparatus of this type one ejection extruder can serve a multiplicity of mold assemblies thereby reducing the capital expense and permitting the extruder to operate more continuously by serially injecting plastic into a series of mold assemblies. In this manner, increased production rates are possible over apparatus in which it is necessary for the extruder to remain idle while the molded parts solidify within the mold cavities. However, it will be appreciated that apparatus which must be intermittently translated from one dwell position to the next is substantially more complex and costly than is the case if it is possible to operate the apparatus continuously. For example, the drive for such apparatus must comprise either a complicated and costly mechanical arrangement such as a Geneva drive, or sophisticated and expensive electrical, hydraulic, or pneumatic drive systems. As a result, equipment of this type incurs both greater initial costs and operating costs than is the case with apparatus which can be operated substantially continuously. In contrast, a constantly rotating turret or table, even though of great weight, may be driven through its endless cycle by a single, relatively simple power train powered by an electric motor which turns at a constant speed. While molding apparatus of this type has been developed previously, for example note U.S. Pat. No. 2,915,957 and British Pat. No. 1,090,913, one problem with such known apparatus is that of maintaining close dimensional tolerances in the articles molded from such apparatus.
Accordingly, it will be appreciated that apparatus for injection molding thermoplastic material into complex parts having close dimensional tolerances at production rates higher than now possible would be extremely advantageous. Moreover, apparatus which is capable of operation continuously, without intermittent motion between the parts, would both simplify the arrangement and lessen the initial and the operating costs thereof. Still further, if the apparatus can mold parts with substantially no runners or sprues which must be separated from the parts after ejection, the apparatus would be even more advantageous. The apparatus would be even more desirable if it ejects the finished parts from the mold cavities in a predetermined orientation which can thereafter be maintained, significantly simplifying subsequent handling of the parts with a substantial resultant cost saving.